Method and Control System for Limiting Vehicle Turning Radius

ABSTRACT

The turning radius of a differentially steered vehicle towing a trailer is controlled when turning so that its turning radius is greater than a minimum allowable turning radius. The turning radius may be autonomously adjusted using a controller to monitor the instantaneous rotational speed differential between the driven wheels and increase or decrease the relative speed between the wheels when the instantaneous rotational speed differential exceeds a threshold rotational speed differential, indicating a turn which is too tight. Alternately, the turning radius may be controlled by the vehicle&#39;s operator, who receives a signal from the controller indicating that the vehicle&#39;s turning radius is less than the minimum allowable. The operator may then take action to enlarge the turning radius using manual controls.

FIELD OF THE INVENTION

This invention relates to differentially steered vehicles and controlsystems for effecting differential steering.

BACKGROUND

Certain self-propelled vehicles, in particular agricultural equipmentsuch as tractors and harvesters, including combines, windrowers, balersand mowers, are advantageously differentially steered. This steeringmethod, wherein wheels or tracks move at different speeds relatively toone another to effect vehicle turning, provides for impressivemaneuverability, and even zero-radius turns for maximum fieldefficiency. However, when differentially steered vehicles tow a trailer,there are limits imposed on the turning radius to avoid contact betweenthe vehicle and its trailer. Trailers may include transport trailers aswell as towed mowers, windrowers and balers and other implements. In atight turn the trailing castors may, for example, contact the tongue ofthe trailer, damaging both. It would be advantageous to provide acontrol system which would allow an operator of such vehicles to avoidturns which are too tight and thereby avoid damage to either or both thevehicle and its trailer.

SUMMARY

One aspect of the invention concerns a method of limiting a turningradius of a differentially steered vehicle when towing a trailer. Thevehicle has a left wheel and a right wheel which are rotatable atdifferent speeds from one another to effect turning of the vehicle. Inone example embodiment the method comprises:

measuring a rotational speed of the left wheel;

measuring a rotational speed of the right wheel;

comparing the rotational speed of the left wheel with the rotationalspeed of the right wheel to establish an instantaneous rotational speeddifferential between the left wheel and the right wheel;

comparing the instantaneous rotational speed differential with athreshold rotational speed differential; wherein if the instantaneousrotational speed differential exceeds the threshold rotational speeddifferential:

increasing the rotational speed of a slower rotating one of the leftwheel and the right wheel; or

decreasing the rotational speed of a faster rotating one of the leftwheel and the right wheel; or

increasing the rotational speed of the slower rotating one of the leftwheel and the right wheel while decreasing the rotational speed of thefaster rotating one of the left wheel and the right wheel.

An example method according to the invention may further comprisesensing that the trailer is coupled to the vehicle. Further by way ofexample, a method may comprise:

comparing the instantaneous rotational speed differential with thethreshold speed differential at a plurality of discrete time intervals;and

increasing the rotational speed of the slower rotating one of the leftwheel and the right wheel unless the instantaneous rotational speeddifferential is equal to or less than the threshold speed differential.

An example method may also comprise:

comparing the instantaneous rotational speed differential with thethreshold speed differential at a plurality of discrete time intervals;and

decreasing the rotational speed of the faster rotating one of the leftwheel and the right wheel unless the instantaneous rotational speeddifferential is equal to or less than the threshold speed differential.

Another example method may comprise:

comparing the instantaneous rotational speed differential with thethreshold speed differential at a plurality of discrete time intervals;and

increasing the rotational speed of the slower rotating one of the leftwheel and the right wheel while decreasing the rotational speed of thefaster rotating one of the left wheel and the right wheel unless theinstantaneous rotational speed differential is equal to or less than thethreshold speed differential.

An example method may include establishing a threshold rotational speeddifferential based upon a parameter selected from the group consistingof a minimum allowable turning radius, a minimum allowable angle betweena direction of motion of the vehicle and a direction of motion of thetrailer, a minimum allowable angle between the vehicle and a tongue ofthe trailer, a wheel base of the vehicle, a wheel base of the trailer, amodel of the vehicle, a model of the trailer, and combinations thereof.

The invention also encompasses a method of alerting an operator of adifferentially steered vehicle towing a trailer while turning at aturning radius that the turning radius is less than a minimum allowableturning radius. The vehicle has a left wheel and a right wheel rotatableat different speeds from one another to effect turning of the vehicle.In one example embodiment the method comprises:

measuring a rotational speed of the left wheel;

measuring a rotational speed of the right wheel;

comparing the rotational speed of the left wheel with the rotationalspeed of the right wheel to establish an instantaneous rotational speeddifferential between the left wheel and the right wheel;

comparing the instantaneous rotational speed differential with athreshold rotational speed differential; wherein

if the instantaneous rotational speed differential exceeds the thresholdrotational speed differential, providing a signal to the operator thatthe turning radius is less than the minimum allowable turning radius.

An example method may further comprise sensing that the trailer iscoupled to the vehicle. Further by way of example, a method maycomprise:

comparing the instantaneous rotational speed differential with thethreshold speed differential at a plurality of discrete time intervals;and

providing the signal to the operator throughout each time interval thatthe instantaneous rotational speed differential is greater than thethreshold speed differential, or:

comparing the instantaneous rotational speed differential with thethreshold speed differential at a plurality of discrete time intervals;and

providing the signal to the operator throughout each the time intervalthat the instantaneous rotational speed differential is equal to orgreater than the threshold speed differential. By way of example thesignal may be an audible signal, a visual signal, a tactile signal orcombinations thereof.

An example method embodiment may further comprise establishing athreshold rotational speed differential based upon a parameter selectedfrom the group consisting of a minimum allowable turning radius, aminimum allowable angle between a direction of motion of the vehicle anda direction of motion of the trailer, a minimum allowable angle betweenthe vehicle and a tongue of the trailer, a wheel base of the vehicle, awheel base of the trailer, a model of the vehicle, a model of thetrailer, and combinations thereof.

The invention also encompasses a control system for steering a vehiclehaving at least a left wheel and a right wheel which are rotatable atdifferent speeds from one another to effect turning of the vehicle. Inone example embodiment the control system comprises:

a controller;

a left wheel rotational speed sensor in communication with thecontroller, the left wheel speed sensor for measuring a rotational speedof the left wheel and sending signals indicative of the rotational speedof the left wheel to the controller;

a right wheel rotational speed sensor in communication with thecontroller, the right wheel speed sensor for measuring a rotationalspeed of the right wheel and sending signals indicative of therotational speed of the right wheel to the controller;

a left wheel actuator in communication with the controller for adjustingthe rotational speed of the left wheel;

a right wheel actuator in communication with the controller foradjusting the rotational speed of the right wheel; wherein

the controller is adapted to:

receive the signals indicative of the left and the right wheelrotational speeds;

compare the left and right wheel rotational speeds with one another toestablish an instantaneous rotational speed differential between theleft wheel and the right wheel; and

adjust the rotational speeds of the left and right wheels using the leftand right wheel actuators and reduce the instantaneous rotational speeddifferential to a value less than or equal to a threshold value pursuantto a control method executed by the controller.

An example control system according to the invention may furthercomprise a trailer sensor in communication with the controller. Thetrailer sensor is adapted to generate signals indicative of the trailerbeing coupled to the vehicle. The controller is adapted to receive thesignals indicative of the trailer being coupled to the vehicle.

In an example embodiment of a control system according to the inventionfor use with the vehicle wherein torque is applied to the left wheel bya left hydraulic pump having a left swashplate, and torque is applied tothe right wheel by a right hydraulic pump having a right swashplate, theleft wheel actuator may comprise a left swashplate actuator incommunication with the controller for adjusting a left swashplate angleof the left swashplate. Further by way of example the right wheelactuator may comprise a right swashplate actuator in communication withthe controller for adjusting a right swashplate angle of the rightswashplate. In the example control system embodiment the controller isadapted to adjust the left and right swashplate angles using the leftand right swashplate actuators to reduce the instantaneous rotationalspeed differential to a value less than or equal to a threshold valuepursuant to a control method executed by the controller.

The invention also includes a differentially steered vehicle. In anexample embodiment the vehicle comprises a chassis having an engine anda plurality of wheels mounted thereon including a left wheel and a rightwheel disposed on opposite sides of the vehicle. A left hydraulic motoris operatively associated with the left wheel and a right hydraulicmotor is operatively associated with the right wheel. A left hydraulicpump is driven by the engine. The left hydraulic pump is operativelyassociated with the left hydraulic motor and has a left swashplate. Aright hydraulic pump is driven by the engine. The right hydraulic pumpis operatively associated with the right hydraulic motor and has a rightswashplate. The example vehicle further comprises a controller. A leftwheel rotational speed sensor is in communication with the controller.The left wheel speed sensor measures a rotational speed of the leftwheel and sends signals indicative of the rotational speed of the leftwheel to the controller. A right wheel rotational speed sensor is incommunication with the controller. The right wheel speed sensor measuresa rotational speed of the right wheel and sends signals indicative ofthe rotational speed of the right wheel to the controller. A leftswashplate actuator is in communication with the controller foradjusting a left swashplate angle of the left swashplate. A rightswashplate actuator is in communication with the controller foradjusting a right swashplate angle of the right swashplate. In anexample embodiment the controller is adapted to:

receive the signals indicative of the left and the right wheelrotational speeds;

compare the left and right wheel rotational speeds with one another toestablish an instantaneous rotational speed differential between theleft wheel and the right wheel; and

adjust the rotational speeds of the left and right wheels using the leftand right wheel actuators and reduce the instantaneous rotational speeddifferential to a value less than or equal to a threshold value pursuantto a control method executed by the controller.

By way of example the vehicle may comprise a tractor or a harvester.

The invention also includes a differentially steered vehicle. In anexample embodiment the vehicle comprises a chassis having an engine anda plurality of wheels mounted thereon including a left wheel and a rightwheel disposed on opposite sides of the vehicle. A left hydraulic motoris operatively associated with the left wheel and a right hydraulicmotor is operatively associated with the right wheel. A left hydraulicpump is driven by the engine. The left hydraulic pump is operativelyassociated with the left hydraulic motor and has a left swashplate. Aright hydraulic pump is driven by the engine. The right hydraulic pumpis operatively associated with the right hydraulic motor and has a rightswashplate. The example vehicle further comprises a controller. A leftwheel rotational speed sensor is in communication with the controller.The left wheel speed sensor measures a rotational speed of the leftwheel and sends signals indicative of the rotational speed of the leftwheel to the controller. A right wheel rotational speed sensor is incommunication with the controller. The right wheel speed sensor measuresa rotational speed of the right wheel and sends signals indicative ofthe rotational speed of the right wheel to the controller. A leftswashplate actuator is in communication with the controller foradjusting a left swashplate angle of the left swashplate. A rightswashplate actuator is in communication with the controller foradjusting a right swashplate angle of the right swashplate. A signaldevice is mounted on the vehicle for signaling an operator of thevehicle. By way of example, the controller is adapted to:

receive the signals indicative of the left and the right wheelrotational speeds;

compare the left and right wheel rotational speeds with one another toestablish an instantaneous rotational speed differential between theleft wheel and the right wheel; and

compare the instantaneous rotational speed differential with a thresholdrotational speed differential; wherein

if the instantaneous rotational speed differential exceeds the thresholdrotational speed differential, provide a signal to the operator that thevehicle is turning at a turning radius less than a minimum allowableturning radius.

By way of example the vehicle may comprise a tractor or a harvester.Further by way of example, the signal device is selected from the groupconsisting of audible alarm devices, visual alarm devices, tactile alarmdevices and combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an example vehicle according to the inventiontowing a trailer;

FIG. 2 is a plan view of an example vehicle according to the inventiontowing a trailer in a turn; and

FIGS. 3 and 4 are flow charts illustrating example methods ofdifferentially steering a vehicle while towing a trailer according tothe invention.

DETAILED DESCRIPTION

FIG. 1 shows an example differentially steered vehicle 10 according tothe invention. Vehicle 10 in this example comprises a tractor 12 towinga transport trailer 14. It is understood that the invention encompassesother types of differentially steered vehicles (tracked as well aswheeled vehicles such as harvesters including self-propelled combines,windrowers, mowers and balers) capable of towing any type of trailerincluding towed wind rowers, mowers, balers and combines. Vehicle 10comprises a chassis 16 on which an engine 18 is mounted. A plurality ofwheels, in this example 4 wheels 20, 22, 24 and 26, are also mounted onchassis 16. Wheels 20 and 22 are free wheeling and caster aboutrespective caster axes 28 to follow vehicle 10 as it turns. Wheels 24(left wheel) and 26 (right wheel) are driven and have angularly fixedaxles, steering being effected by applying different torque to one wheelthan the other thereby causing the wheel with the greater applied torqueto rotate faster than the wheel having less applied torque, the turnbeing in the direction of the slower rotating wheel.

Torque is applied to the left wheel 24 by a left hydraulic motor 30operatively associated with the left wheel, and torque is applied to theright wheel 26 by a right hydraulic motor 32 operatively associated withthe right wheel, both motors mounted on the chassis 16. The lefthydraulic motor 30 is driven by a left hydraulic pump 34 operativelyassociated with the left hydraulic motor and the right hydraulic motor32 is driven by a right hydraulic pump 36 operatively associate with theright hydraulic motor. Both hydraulic pumps 34 and 36 are driven byengine 18 through a gear box 38 and may comprise axial piston pumps asin this example.

As is well understood, the output (pressure, volume rate of flow) froman axial piston pump is controlled by adjusting the angle of the pump'sswashplate with respect to the pump shaft's axis of rotation. Largerswashplate angles produce greater pump output. Vehicle 10 comprises aleft swashplate 40 in the left hydraulic pump 34 and a right swashplate42 in the right hydraulic pump 36. The left swashplate 40 is adjustablethrough a left swashplate angle 44 and the right swashplate 42 isadjustable through a right swashplate angle 46. In this exampleembodiment the left swashplate angle 44 is adjusted by a left swashplateactuator 48 and the right swashplate angle 46 is adjusted by a rightswashplate actuator 50. Actuators 48 and 50 may be hydraulic actuatorsas in this example, driven by an actuator pump 52 by engine 18 throughgear box 38.

Vehicle 10 is steered differentially by adjusting the left and rightswashplate angles 44 and 46 of the left and right hydraulic pumps 34 and36. When the left and right swashplate angles are equal and both pumps34 and 36 are driven at the same speed, equal torque will be applied tothe left and right wheels 24 and 26 via their respective left and righthydraulic motors 30 and 32. The left and right wheels 24 and 26 have thesame rotational speed about their axes of rotation and the vehicle 10will move along a straight line. The speed of vehicle 10 is proportionalto the swashplate angle, with greater swashplate angles producing higherspeeds. To turn to the left, the swashplate angles 44 and 46 areadjusted with respect to one another so that the right swashplate angle46 is greater than the left swashplate angle 44. More hydraulic oilflows to the right hydraulic motor 32 and thus more torque is applied tothe right wheel 26 which causes the right wheel 26 to rotate faster thanthe left wheel 24 and vehicle 10 consequently turns to the left. To turnto the right, the swashplate angles are adjusted with respect to oneanother so that the left swashplate angle 44 is greater than the rightswashplate angle 46. More hydraulic oil flows to the left hydraulicmotor 30 and thus more torque is applied to the left wheel 24 whichcauses the left wheel 24 to rotate faster than the right wheel 26 andconsequently vehicle 10 turns to the right. The turning radius of thevehicle is proportional to the difference between the rotational speedof the left and right wheels, known as the instantaneous rotationalspeed differential.

Vehicle 10 further comprises a control system 54. Control system 54permits differential steering of vehicle 10 according to a method which,as shown in FIG. 2, limits the vehicle's turning radius 56 to ensure itis greater than or equal to a minimum allowable turning radius 56 a. Themethod, examples of which are described below, is advantageously invokedwhen vehicle 10 is towing trailer 14. Differentially steered vehicles 10are capable of very tight turns, even down to zero-radius turns whenwheels 24 and 26 are rotated in opposite directions. While thiscapability provides superior maneuverability for maximum fieldefficiency, it can have adverse effects when a trailer is towed. Tightturns can result in contact between the vehicle 10 and trailer 14, forexample, between caster wheel 20 and the tongue 58 of trailer 14 in FIG.2. To avoid such contact, and the potential damage to both vehicle andtrailer, it is advantageous to employ an example control system andsteering method according to the invention.

As shown in FIG. 1, control system 54 comprises a controller 60, forexample a programmable logic controller or other microprocessor basedcomputer mounted on chassis 16. Resident software on the controllerprovides algorithms to execute the differential method of steeringaccording to the invention. The controller 60 may be in communicationwith a trailer sensor 62 which detects the presence of trailer 14coupled to vehicle 10. Trailer sensor 62 generates signals indicative ofthe trailer coupled to the vehicle which it transmits to the controller60. Communication between the controller 60 and the trailer sensor 62may be via wires or wirelessly via radio frequency transmitters. Exampletrailer sensors include a micro switch which closes when a tow hitch onvehicle 10 is deployed or the tongue 58 is coupled to the hitch, or atrailer lighting harness which closes a circuit to the controller whenit is connected to the vehicle electrical system. Controller 60 isadapted to receive signals from the trailer sensor 62 and invoke themethod of differential steering according to the invention

Control system 54 further comprises a left wheel rotational speed sensor64 and a right wheel rotational speed sensor 66. These sensors areoperatively associated with respective wheels 24 and 26 and measure therespective rotational speeds of each wheel. Rotational speed sensors 64and 66 also generate signals indicative of the rotational speed ofwheels 24 and 26 and transmit the signals to controller 60. Transmissionof the signals may be via wire or wirelessly via radio frequencytransmitters. Controller 60 is adapted to receive the signals from therotational speed sensors 64 and 66 and compare the left and right wheelrotational speeds to establish the instantaneous rotational speeddifferential between them. The turning radius 56 of vehicle 10 (see FIG.2) will be a function of the instantaneous rotational speeddifferential, with a larger rotational speed differential correspondingto a tighter turn and a smaller turning radius.

As further shown in FIG. 1, control system 54 also comprises a leftwheel actuator 68 and a right wheel actuator 70. Actuators 68 and 70 arein communication with controller 60, which is adapted to transmitcommand signals to the actuators allowing the controller to adjust(increase or decrease) the rotational speeds of the left and rightwheels 24 and 26 and thereby determine the turning radius 56 of vehicle10 (see also FIG. 2). In this example embodiment the left wheel actuator68 comprises the left swashplate actuator 48 and the right wheelactuator 70 comprises the right swashplate actuator 50. The swashplateactuators 48 and 50 respectively control the swashplate angles 44 and 46and thus control the respective rotational speeds of the right and leftwheels 24 and 26. The controller 60 is adapted to adjust the left andright swashplate angles 44 and 46 via the actuators and thereby adjustthe instantaneous rotational speed differential, for example, to reduceit to a value less than or equal to a threshold value which correspondsto a desired turning radius greater than the minimum allowable turningradius 56 a of the vehicle 10.

An example steering control method which limits a turning radius of adifferentially steered vehicle 10 when towing a trailer 14 isillustrated in FIGS. 2 and 3. As shown in the flow chart of FIG. 3, theexample method 72 may include sensing that trailer 14 is coupled tovehicle 10 (74). This step is conveniently executed using the trailersensor 62. When controller 60 receives a signal from the trailer sensor62 the controller invokes the steering method 72, the method otherwisenot being needed or desired in the absence of towed trailer. Thecontroller next measures the rotational speed of the left and rightwheels 24 and 26 (76) using respective left and right wheel speedsensors 64 and 66 and compares these rotational speeds to one another toestablish the instantaneous rotational speed differential between theleft and right wheels 24 and 26 (78). Controller 60 next compares theinstantaneous rotational speed differential with a threshold rotationalspeed differential (80). This threshold rotational speed differentialcorresponds to a minimum allowable turning radius 56 a (see FIG. 2). Thethreshold rotational speed differential will be different for differentvehicles and trailers and may be established based upon one or moreparameters such as an experimentally determined minimum allowableturning radius 56 a, a maximum allowable angle 82 between a direction ofmotion 84 of the vehicle 10 and a direction of motion 86 of the trailer14, a minimum allowable angle 88 between the vehicle 10 and the tongue58 of the trailer 14, a wheel base 90 of the vehicle 10, a wheel base 92of the trailer 14, the type (model) of the vehicle 10, the type (model)of the trailer 14, and combinations of these parameters.

If the instantaneous rotational speed differential established by thecontroller is greater than the threshold rotational speed differential(94) then the vehicle 10 is in danger of turning tighter than permittedwhile towing trailer 14 and the controller may act in at least one ofthree ways: 1) by increasing the rotational speed of the slower rotatingwheel (96); 2) decreasing the rotational speed of the faster rotatingwheel (98); or 3) increasing the rotational speed of the slower rotatingwheel while simultaneously decreasing the rotational speed of the fasterrotating wheel (100). Once a turning radius larger than or equal to theminimum allowable turning radius 56 a is established (as indicated bythe instantaneous rotational speed differential being equal to or lessthan the threshold rotational speed differential) the controller ceaseschanging the rotational speed of the wheels. It is further advantageousfor controller 60 to compare the instantaneous rotational speeddifferential with the threshold speed differential at a plurality ofdiscrete time intervals (102) and thereby constantly monitor the turningradius of the vehicle 10 and take appropriate action as theinstantaneous rotational speed differential changes over time while thevehicle is moving.

FIG. 4 illustrates another embodiment of a method 104 for limiting theturning radius of a vehicle 10. However, instead of having thecontroller 60 directly adjust the rotational speed of the wheels 24 and26, the method 104 alerts an operator of the vehicle that the vehicle isturning at a turning radius which is less than the minimum allowableturning radius 56 a. Method 104 measures the rotational speeds of theleft and right wheels (76), establishes the instantaneous rotationalspeed differential (78), compares the instantaneous rotational speeddifferential with the threshold rotational speed differential (80) andthen if the instantaneous rotational speed differential exceeds thethreshold rotational speed differential the controller provides a signalto the operator indicating that the instantaneous turning radius is lessthan the minimum allowed turning radius 56 a (106). The signal may be anaudible signal, a visual signal, a tactile signal, or any combination ofsuch signals. To accommodate method 104 the controller 60 is adapted tocommunicate with one or more alarm devices 108 mounted on vehicle 10,preferably within its control cab (not shown). The alarm devices 108 mayproduce audible signals, such as a horn or siren, a visual signal, forexample on a display screen, or a tactile signal, such as a vibrationtransmitted to the operator through a seat or a control device such as asteering wheel.

It is expected that vehicles, such as tractors and harvesters includingself-propelled windrowers, mowers, balers and combines, which towtrailers such as transport trailers, mowers, and windrowers, will beable to better avoid damage to both the vehicle and the trailer whileturning when methods and vehicles according to the invention areimplemented.

What is claimed is:
 1. A method of limiting a turning radius of adifferentially steered vehicle when towing a trailer, said vehiclehaving a left wheel and a right wheel being rotatable at differentspeeds from one another to effect turning of said vehicle, said methodcomprising: measuring a rotational speed of said left wheel; measuring arotational speed of said right wheel; comparing said rotational speed ofsaid left wheel with said rotational speed of said right wheel toestablish an instantaneous rotational speed differential between saidleft wheel and said right wheel; comparing said instantaneous rotationalspeed differential with a threshold rotational speed differential;wherein if said instantaneous rotational speed differential exceeds saidthreshold rotational speed differential: increasing said rotationalspeed of a slower rotating one of said left wheel and said right wheel;or decreasing said rotational speed of a faster rotating one of saidleft wheel and said right wheel; or increasing said rotational speed ofsaid slower rotating one of said left wheel and said right wheel whiledecreasing said rotational speed of said faster rotating one of saidleft wheel and said right wheel.
 2. The method according to claim 1,further comprising sensing that said trailer is coupled to said vehicle.3. The method according to claim 1, further comprising: comparing saidinstantaneous rotational speed differential with said threshold speeddifferential at a plurality of discrete time intervals; and increasingsaid rotational speed of said slower rotating one of said left wheel andsaid right wheel unless said instantaneous rotational speed differentialis equal to or less than said threshold speed differential.
 4. Themethod according to claim 1, further comprising: comparing saidinstantaneous rotational speed differential with said threshold speeddifferential at a plurality of discrete time intervals; and decreasingsaid rotational speed of said faster rotating one of said left wheel andsaid right wheel unless said instantaneous rotational speed differentialis equal to or less than said threshold speed differential.
 5. Themethod according to claim 1, further comprising: comparing saidinstantaneous rotational speed differential with said threshold speeddifferential at a plurality of discrete time intervals; and increasingsaid rotational speed of said slower rotating one of said left wheel andsaid right wheel while decreasing said rotational speed of said fasterrotating one of said left wheel and said right wheel unless saidinstantaneous rotational speed differential is equal to or less thansaid threshold speed differential.
 6. The method according to claim 1,further comprising establishing a threshold rotational speeddifferential based upon a parameter selected from the group consistingof a minimum allowable turning radius, a minimum allowable angle betweena direction of motion of said vehicle and a direction of motion of saidtrailer, a minimum allowable angle between said vehicle and a tongue ofsaid trailer, a wheel base of said vehicle, a wheel base of saidtrailer, a model of said vehicle, a model of said trailer, andcombinations thereof.
 7. A method of alerting an operator of adifferentially steered vehicle towing a trailer while turning at aturning radius that said turning radius is less than a minimum allowableturning radius, said vehicle having a left wheel and a right wheel beingrotatable at different speeds from one another to effect turning of saidvehicle, said method comprising: measuring a rotational speed of saidleft wheel; measuring a rotational speed of said right wheel; comparingsaid rotational speed of said left wheel with said rotational speed ofsaid right wheel to establish an instantaneous rotational speeddifferential between said left wheel and said right wheel; comparingsaid instantaneous rotational speed differential with a thresholdrotational speed differential; wherein if said instantaneous rotationalspeed differential exceeds said threshold rotational speed differential,providing a signal to said operator that said turning radius is lessthan said minimum allowable turning radius.
 8. The method according toclaim 7, further comprising sensing that said trailer is coupled to saidvehicle.
 9. The method according to claim 7, further comprising:comparing said instantaneous rotational speed differential with saidthreshold speed differential at a plurality of discrete time intervals;and providing said signal to said operator throughout each said timeinterval that said instantaneous rotational speed differential isgreater than said threshold speed differential.
 10. The method accordingto claim 7, further comprising: comparing said instantaneous rotationalspeed differential with said threshold speed differential at a pluralityof discrete time intervals; and providing said signal to said operatorthroughout each said time interval that said instantaneous rotationalspeed differential is equal to or greater than said threshold speeddifferential.
 11. The method according to claim 7, wherein said signalis selected from the group consisting of an audible signal, a visualsignal, a tactile signal and combinations thereof.
 12. The methodaccording to claim 7, further comprising establishing a thresholdrotational speed differential based upon a parameter selected from thegroup consisting of a minimum allowable turning radius, a minimumallowable angle between a direction of motion of said vehicle and adirection of motion of said trailer, a minimum allowable angle betweensaid vehicle and a tongue of said trailer, a wheel base of said vehicle,a wheel base of said trailer, a model of said vehicle, a model of saidtrailer, and combinations thereof.
 13. A control system for steering avehicle having at least a left wheel and a right wheel being rotatableat different speeds from one another to effect turning of said vehicle,said control system comprising: a controller; a left wheel rotationalspeed sensor in communication with said controller, said left wheelspeed sensor for measuring a rotational speed of said left wheel andsending signals indicative of said rotational speed of said left wheelto said controller; a right wheel rotational speed sensor incommunication with said controller, said right wheel speed sensor formeasuring a rotational speed of said right wheel and sending signalsindicative of said rotational speed of said right wheel to saidcontroller; a left wheel actuator in communication with said controllerfor adjusting said rotational speed of said left wheel; a right wheelactuator in communication with said controller for adjusting saidrotational speed of said right wheel; wherein said controller is adaptedto: receive said signals indicative of said left and said right wheelrotational speeds; compare said left and right wheel rotational speedswith one another to establish an instantaneous rotational speeddifferential between said left wheel and said right wheel; and adjustsaid rotational speeds of said left and right wheels using said left andright wheel actuators and reduce said instantaneous rotational speeddifferential to a value less than or equal to a threshold value pursuantto a control method executed by said controller.
 14. The control systemaccording to claim 13, further comprising a trailer sensor incommunication with said controller, said trailer sensor adapted togenerate signals indicative of said trailer being coupled to saidvehicle, said controller being adapted to receive said signalsindicative of said trailer being coupled to said vehicle.
 15. Thecontrol system according to claim 13 for use with said vehicle whereintorque is applied to said left wheel by a left hydraulic pump having aleft swashplate, torque is applied to said right wheel by a righthydraulic pump having a right swashplate, wherein: said left wheelactuator comprises a left swashplate actuator in communication with saidcontroller for adjusting a left swashplate angle of said leftswashplate; said right wheel actuator comprises a right swashplateactuator in communication with said controller for adjusting a rightswashplate angle of said right swashplate; wherein said controller isadapted to adjust said left and right swashplate angles using said leftand right swashplate actuators to reduce said instantaneous rotationalspeed differential to a value less than or equal to a threshold valuepursuant to a control method executed by said controller.
 16. Adifferentially steered vehicle, said vehicle comprising: a chassishaving an engine and a plurality of wheels mounted thereon including aleft wheel and a right wheel disposed on opposite sides of said vehicle;a left hydraulic motor operatively associated with said left wheel; aright hydraulic motor operatively associated with said right wheel; aleft hydraulic pump driven by said engine, said left hydraulic pumpoperatively associated with said left hydraulic motor and having a leftswashplate; a right hydraulic pump driven by said engine, said righthydraulic pump operatively associated with said right hydraulic motorand having a right swashplate; a controller; a left wheel rotationalspeed sensor in communication with said controller, said left wheelspeed sensor for measuring a rotational speed of said left wheel andsending signals indicative of said rotational speed of said left wheelto said controller; a right wheel rotational speed sensor incommunication with said controller, said right wheel speed sensor formeasuring a rotational speed of said right wheel and sending signalsindicative of said rotational speed of said right wheel to saidcontroller; a left swashplate actuator in communication with saidcontroller for adjusting a left swashplate angle of said leftswashplate; a right swashplate actuator in communication with saidcontroller for adjusting a right swashplate angle of said rightswashplate; wherein said controller is adapted to: receive said signalsindicative of said left and said right wheel rotational speeds; comparesaid left and right wheel rotational speeds with one another toestablish an instantaneous rotational speed differential between saidleft wheel and said right wheel; and adjust said rotational speeds ofsaid left and right wheels using said left and right wheel actuators andreduce said instantaneous rotational speed differential to a value lessthan or equal to a threshold value pursuant to a control method executedby said controller.
 17. The vehicle according to claim 16, wherein saidvehicle comprises a tractor.
 18. The vehicle according to claim 16,wherein said vehicle comprises a harvester.
 19. A differentially steeredvehicle, said vehicle comprising: a chassis having an engine and aplurality of wheels mounted thereon including a left wheel and a rightwheel disposed on opposite sides of said vehicle; a left hydraulic motoroperatively associated with said left wheel; a right hydraulic motoroperatively associated with said right wheel; a left hydraulic pumpdriven by said engine, said left hydraulic pump operatively associatedwith said left hydraulic motor and having a left swashplate; a righthydraulic pump driven by said engine, said right hydraulic pumpoperatively associated with said right hydraulic motor and having aright swashplate; a controller; a left wheel rotational speed sensor incommunication with said controller, said left wheel speed sensor formeasuring a rotational speed of said left wheel and sending signalsindicative of said rotational speed of said left wheel to saidcontroller; a right wheel rotational speed sensor in communication withsaid controller, said right wheel speed sensor for measuring arotational speed of said right wheel and sending signals indicative ofsaid rotational speed of said right wheel to said controller; a leftswashplate actuator in communication with said controller for adjustinga left swashplate angle of said left swashplate; a right swashplateactuator in communication with said controller for adjusting a rightswashplate angle of said right swashplate; a signal device mounted onsaid vehicle for signaling an operator of said vehicle; wherein saidcontroller is adapted to: receive said signals indicative of said leftand said right wheel rotational speeds; compare said left and rightwheel rotational speeds with one another to establish an instantaneousrotational speed differential between said left wheel and said rightwheel; and compare said instantaneous rotational speed differential witha threshold rotational speed differential; wherein if said instantaneousrotational speed differential exceeds said threshold rotational speeddifferential, provide a signal to said operator that said vehicle isturning at a turning radius less than a minimum allowable turningradius.
 20. The vehicle according to claim 19, wherein said vehiclecomprises a tractor.
 21. The vehicle according to claim 19, wherein saidvehicle comprises a harvester.
 22. The vehicle according to claim 19,wherein said signal device is selected from the group consisting ofaudible alarm devices, visual alarm devices, tactile alarm devices andcombinations thereof.